1. Field of Invention
The present invention is related to a method for producing a wear-resistant iron-based sintered alloy used, for example, for a valve seat of an internal combustion engine.
2. Description of Related Arts
Mo, W, V, Nb, Ta and the like are the alloying elements used as the additives of an iron-based sintered alloy used for the valve seat and the like. In most cases, they are in the form of hard particles, such as ferro-alloy, carbide, and composite alloys, and are mixed in the raw-material powder. The hard particles are therefore dispersed in the sintered alloy. In the case of the use of hard particles, since Cr, Mo, W. V, Nb and Ta are hard to diffuse deeply in the iron matrix, not the entire matrix and only the circumference of the hard particles is solution strengthened. The matrix is therefore mainly dispersion-strengthened. In other words, it cannot be expected that Cr, Mo, W, V, Nb and Ta will be dissolved and alloyed with the entire matrix.
On the other hand, Cu is easy to diffuse in the iron matrix and noticeably strengthens the iron matrix due to fine solution and precipitation therein. The Cu, which is finely precipitated in a valve seat, is effective for buffering the impact on the valve seat, which is struck by the opposed material, i.e., the valve. The Cu, which is a soft minority phase of the valve seat, is effective for mitigating its attacking action against the valve, when the valve seat and valve are subjected to wear under tapping. The present applicants filed Japanese Patent Applications Nos. Showa 63-255363 and Heisei 1-183073. In these patent applications, Cu is once dissolved as a solute element of the matrix during the production process of the sintered alloy and is then uniformly precipitated by heat treatment.
The ordinary Cu powders, namely the atomized and crushed Cu powders, are used as the Cu source in the above described method for producing the sintered alloy. When the ordinary Cu powders are mixed in the raw-material powder, the Cu particles coagulate to form coarse Cu lumps from a few tens to a few hundreds .mu.m in size. When the Cu is then dissolved during the sintering process, coarse pores may be formed at the portions where the Cu particles have been present.
Along with recent enhancement of the performance of the automobile engines, the load applied to the sliding surface of a valve seat, is subsequently increasing. This is one of the applications of the above described wear-resistant iron-based sintered alloy. It becomes, therefore, necessary to highly densify and hence increase the strength of such an alloy. Meanwhile, a valve seat must be made so thin as to enhance the cooling efficiency and to lessen the weight thereof. The necessity of the strength-enhancement therefore arises.